The present invention relates to trays and more particularly to methods and apparatus for producing lined trays.
Many products are merchandised in closed trays in high volumes but at a relatively low profit margin on a per tray basis. Examples of high volume, low margin products are typical frozen food products such as vegetable entrees or complete dinners which are packaged in closed trays before freezing. Such products are maintained in a frozen state until reheated by a consumer in a conventional or microwave oven.
The packaging operation by which the food products are sealed in individual trays should be highly automated and should employ low cost materials to reduce the packaging costs without reducing the integrity of the package seal. Paperboard is strong and relatively inexpensive, and therefore highly suitable for many packaging applications.
U.S. Pat. No. 3,932,105, issued Jan. 13, 1976, and assigned to the assignee of the present invention, discloses equipment and a process for lining an erected paperboard tray with a continuous sheet of film. In a preferred embodiment, the film is heated and then drawn into intimate contact with the interior tray walls by applying a vacuum to the exterior bottom wall and exterior side walls of the tray. While the equipment and process disclosed in this patent produce leak-free, low-cost lined trays, there is room for improvement.
The trays produced in accordance with the patent have a liner which conforms to the interior tray walls, forming a single product-receiving volume. The erected (and lined) paperboard tray tends to have angular corners. When such a tray is heated in a microwave oven, the food in the corners is exposed to relatively greater amounts of microwave energy than the food in the rest of the carton, resulting in uneven heating.
While a press-formed tray might be employed to avoid the angular corners of an erected tray, press formed trays have other disadvantages. Graphic material printed on the tray tends to be distorted. The process itself is at least initially more costly since each set of press-forming tooling is custom made for the particular tray shap to be formed. Press formed trays also suffer from the disadvantage that such trays are inherently difficult to seal.